Photocatalysts provide for catalysis of chemical reactions when irradiated by electromagnetic radiation. One advantage of photocatalysts is environmental stability, since they are relatively inert until subjected to irradiation. Photocatalysts have been investigated for applications in a variety of areas, including environmental remediation. Stable catalysts that promote oxidation reactions can oxidize pollutants in air and in water, including inorganic pollutants, organic pollutants and microorganisms. By breaking the pollutants down into substances that are less harmful, the need for storage or disposal of pollutants that have been extracted from air or water can be minimized.
Titanium oxide (TiO2) in the anatase structure type is an example of a photocatalyst that is useful in many different applications. Titanium oxide is a semiconductor that is stable in a wide range of environmental conditions and that promotes oxidation-reduction (redox) reactions when irradiated with ultraviolet light. Systems utilizing titanium oxide and ultraviolet radiation have been used for commercial water purification. In addition, titanium oxide has been investigated and/or used for applications including air purification, hydrogen production by the photochemical splitting of water, nitrogen fixation, odor control, antimicrobial surface coatings, and self-cleaning surface coatings.
One drawback to the use of titanium oxide as a photocatalyst is the requirement for irradiation by ultraviolet light, due to its large band gap. Since ultraviolet radiation is only a small portion of solar radiation once it has passed though the atmosphere, titanium oxide has a low photon yield when exposed to sunlight. Doping of titanium oxide with other elements can provide for a narrowing of the band gap, allowing for increased reactivity under visible light. These doped materials do not necessarily provide for increased oxidation of pollutants under visible light, however. Possible explanations for the low oxidation efficiency include rapid recombination of the electron-hole pairs before oxidation can occur, and the short lifetimes of the charge carrying dopants. In addition, doped titanium oxide typically is expensive to produce and is difficult to obtain in large enough quantities for use in environmental remediation facilities.
A stable photocatalyst that can efficiently promote redox reactions under visible light would be beneficial. It would be desirable to form such a photocatalyst by a method that is relatively inexpensive and that can be readily scaled up to produce large quantities.